Especially in this age of Scribunto, it sometimes happens that revisions will be imported that end up breaking templates or causing other problems. E.g., you might import a bunch of templates from Wikipedia and then import some from other wikis and find that you're getting script errors because you overwrote something you didn't mean to. It can be hard to sort out which templates are causing what problems. Re-importing from Wikipedia doesn't necessarily help because those revisions might be older than the revisions that are causing the problems.
It would be helpful to be able to do a query and find out what pages' current revisions (page.page_latest) were imported from what wikis. That way, one could sort out what needs to be reverted. Therefore, I propose one of two solutions for storing in the database the data that is in <sitename> in the XML file:
(1) Add a revision.rev_imported field to store the name of the source wiki of imported revisions.
(2) Add a new table that will store the same data as in option 1.
Option 1 makes sense if a large proportion of the revisions on the wiki will have been imported from other wikis. Option 2 makes sense if the proportion is lower. I suspect that people are going to want to go with option 2.